Coral reefs consist of huge communities of organisms, which live on or associated with huge limestone structures formed by animals called coral polyps. Geological evidence suggests they have been around for at least 240 million years. Existing established reefs are 5,000-10,000 years old.
Coral polyps are invertebrate animals, ranging from pinhead size to several centimetres across, and are related to jellyfish.
Hard corals produce the limestone which forms the “skeleton” of the reef, and rely partly on sunlight-gathering algae for oxygen and sustenance, which is why hard corals are usually found near the water surface in clear, clean water. The coral polyps usually stay within the protection of their limestone “shell” though the day, emerging at night to feed. When the polyps die off, the limestone that remains forms the skeleton for the next generation of polyps, and so the reef grows.
Hard corals come in all kinds of shapes and sizes. Examples include “staghorn coral” and “brain coral”.
Soft corals, which often resemble plants, do not produce the limestone skeleton, and can be found both in association with hard corals and on their own.
Coral reefs support around a quarter of all known marine species, including thousands of types of corals, fish, and other animals and plants. They are found in more than 100 countries, mostly in the warm tropical waters of the Pacific Ocean, the Caribbean and Red Seas, and the Arabian Gulf. Worldwide, coral reefs cover around 300,000 square km.
Corals breed using various methods. Some types release their eggs and sperm into the water on a particular night of the year, producing spectacular mass coral spawning events.
Coral reefs come in various categories, including:
- Barrier reefs: These run parallel to the shore, and sometimes break the surface, forming a “barrier” between the shore and the open sea. Australia’s Great Barrier Reef is the best known example.
- Fringing reefs: These are the most common types of reef, and occur near the coastline of islands and continents.
- Atolls: Atolls are circular reefs enclosing a central lagoon, and usually occur in the middle of the sea. The Maldives in the Indian Ocean consist of a series or chain of atolls.
- Patch reefs: These occur in “patches”, growing up from the sea bed, and rarely reach the surface. They often occur between fringing and barrier reefs.